@@ -455,3 +455,17 @@ test("Container#atPosition comma in second selector pseudo", (t) => {
     t.deepEqual(node.toString(), ":matches(ol, ul)");
   });
 });
+
+// Regression for #320/#321/#322: passing an array to replaceWith() stores a
+// raw Array (not a Node) inside `nodes`. Serialization must tolerate it, as it
+// did before the recursion fix — this is how Tailwind v3 expands `:merge()`
+// for every group-* / peer-* variant.
+test("Container#toString tolerates a raw array child from replaceWith(array)", (t) => {
+  const ast = parser().astSync(":merge(.group):hover .foo");
+  ast.walkPseudos((p) => {
+    if (p.value === ":merge") {
+      p.replaceWith(p.nodes); // array passed directly, not spread
+    }
+  });
+  t.deepEqual(ast.toString(), ".group:hover .foo");
+});
@@ -337,6 +337,17 @@ export default class Container extends Node {
   }
 
   _stringify(options, depth, max) {
-    return this.map((child) => child._stringify(options, depth, max)).join("");
+    return this.map((child) => this._stringifyChild(child, options, depth, max)).join("");
+  }
+
+  // Serialize a child node. Historically `toString` used `this.map(String)`,
+  // which leniently coerced anything — including raw arrays inserted via
+  // `replaceWith(array)` / `insertBefore` / `insertAfter` (e.g. Tailwind's
+  // `:merge()` expansion). Fall back to `String(child)` for values that are not
+  // parser nodes so that behaviour is preserved.
+  _stringifyChild(child, options, depth, max) {
+    return typeof child._stringify === "function"
+      ? child._stringify(options, depth, max)
+      : String(child);
   }
 }
@@ -12,7 +12,9 @@ export default class Pseudo extends Container {
       throw new Error(`Cannot serialize selector: nesting depth exceeds the maximum of ${max}.`);
     }
     let params = this.length
-      ? "(" + this.map((child) => child._stringify(options, depth + 1, max)).join(",") + ")"
+      ? "(" +
+        this.map((child) => this._stringifyChild(child, options, depth + 1, max)).join(",") +
+        ")"
       : "";
     return [this.rawSpaceBefore, this.stringifyProperty("value"), params, this.rawSpaceAfter].join(
       "",
 
@@ -9,7 +9,7 @@ export default class Root extends Container {
 
   _stringify(options, depth, max) {
     let str = this.reduce((memo, selector) => {
-      memo.push(selector._stringify(options, depth, max));
+      memo.push(this._stringifyChild(selector, options, depth, max));
       return memo;
     }, []).join(",");
     return this.trailingComma ? str + "," : str;
Original file line number	Diff line number	Diff line change
`@@ -337,6 +337,17 @@ export default class Container extends Node {`
`337`	`337`	`}`
`338`	`338`
`339`	`339`	`_stringify(options, depth, max) {`
`340`		`- return this.map((child) => child._stringify(options, depth, max)).join("");`
	`340`	`+ return this.map((child) => this._stringifyChild(child, options, depth, max)).join("");`
	`341`	`+ }`
	`342`	`+`
	`343`	+ // Serialize a child node. Historically `toString` used `this.map(String)`,
	`344`	`+ // which leniently coerced anything — including raw arrays inserted via`
	`345`	+ // `replaceWith(array)` / `insertBefore` / `insertAfter` (e.g. Tailwind's
	`346`	+ // `:merge()` expansion). Fall back to `String(child)` for values that are not
	`347`	`+ // parser nodes so that behaviour is preserved.`
	`348`	`+ _stringifyChild(child, options, depth, max) {`
	`349`	`+ return typeof child._stringify === "function"`
	`350`	`+ ? child._stringify(options, depth, max)`
	`351`	`+ : String(child);`
`341`	`352`	`}`
`342`	`353`	`}`